I brought my hibiscus in for winter. It did fine up to early January, and it even bloomed weekly. Now, however, the leaves are falling off, even green ones, and almost half the leaves are gone.
Hibiscus Leaves Falling Off
Certified GKH Gardening Expert
This is pretty common for hibiscus and is normal. The lack of sunlight causes them to head into dormancy and drop leaves. You can try to stop it by adding supplemental light from a lamp, or just let it go semi-dormant until spring, when it will perk up again on its own.
Semi-dormant simply means to let it stay in a state where it loses its leaves somewhat, but a few stay on. It does not look very pretty, but it is difficult to keep hibiscus from doing this anyway. I would cut back the water some during this time, only watering when the soil is dry to the touch.